The invention relates to an electron tube comprising an electrode of pyrolytic graphite, which is a hollow body of revolution and which has an open end at which it is secured to a cylindrical electrode support.
Electrodes of pyrolytic graphite have been used in transmitting tubes for a long time.
Such an electron tube is known from, for example, U.S. Pat. No. 4,229,674, in which the grid electrode of pyrolytic graphite is secured to the electrode support by means of a number of screws which are spaced equidistantly around the grid. In order to ensure proper electrical contact between these parts a large number of screws is necessary. In a transmitting tube, the screws are often located in a region of high fieldstrength so that additional steps are necessary to preclude disturbance of the electric field.
British Patent Specification No. GB 1,514,591 describes how this problem is solved by means of two conical auxiliary rings which are secured to each other by means of screws, soldering or welding and between which the grid electrode of pyrolytic graphite is clamped. However, this is relatively complicated and time-consuming to manufacture and it cannot readily be used in existing types of tubes.
German Patent Specification 11 94 988 corresponding to British Patent Specification No. 1,011,587, describes a grid electrode of pyrolytic graphite which is secured to an auxiliary ring of pure graphite by means of titanium carbide, zirconium carbide or pure titanium. A disadvantage of this method is that it employs ordinary graphite which may adversely affect the dielectric strength of the tube.
French Patent Application No. 2,439,474, which is open to public inspection, discloses a grid electrode of pyrolytic graphite, which is cast in a trough-shaped element of metal (e.g. molybdenum, tungsten or an alloy of iron-nickel and cobalt) or of graphite by means of a soldering material (e.g. copper, gold or a copper-silver alloy). A disadvantage of this method is that a high grid load causes the soldering material to become so hot that it evaporates.
European Patent Specification No. 0,010,128 B discloses a cathode having a body of pyrolytic graphite, which is covered with a layer of an emitting material. When securing this cathode the same problems occur as with the grids described above.